Video card driver crashed

i was sitting on my computer running world of warcraft and ive mozilla firefox as well and all of a sudden my screen goes black and then it resets and it comes up with graphics card driver has crashed and it reloaded it (not exactly what it said but along those lines) this isnt the first time ive seen it it happened before when my i burtout my previous graphics card and im worried thats what ive done again if it is that i at least know what i have to do to fix it but if it isnt does any one know how to
im running a NVIDIA geforce GTS 450 IGB GDDRS